Rhizaria

Abstract

Rhizaria are a diverse group of amoeboid protists characterised by the presence of pseudopodia, cytoplasmic extensions used for locomotion and/or predation. Pseudopodial morphology includes broad and fan‐shaped anterior extensions, finger‐like tubular extensions or a web‐like network. Some Rhizaria build complex tests using inorganic/organic material or a variety of minerals (e.g. opaline, calcium carbonate). The shape of the pseudopodia, and the morphology of the enclosing test or skeleton, when present, are major defining taxonomic characteristics. They are distributed worldwide from surface to deep waters, but also in freshwater and terrestrial ecosystems. Therefore, Rhizaria are important marine and terrestrial protists at the base of food webs and thus provide a major link in the transfer of energy to higher‐order consumers. While most are capable of preying on diverse organisms (from inorganic marine snow to bacteria and metazoans), others are obligatory parasites (of land plants or marine invertebrates), and only a few contribute to primary production via photosymbiosis (e.g. Radiolaria) or acquired phototrophy (e.g. Chlorarachnea). Key Concepts The shape and organisation of pseudopodia were among major characteristics used to classify members of the Rhizaria. However, pseudopodial shape is not a conserved feature; and additional evidence, including the fine structure and molecular genetic data, is needed to create a natural classification scheme. The diversity of Rhizaria is broad, including a wide range in size from small naked amoebae (several microns) to giant single cells (up to several centimetres or larger for Xenophyophores). A vast majority of Rhizaria are heterotrophs, feeding on a large diversity of prey, but two groups (Chlorarachnea and Paulinella ) include photosynthetic species. Many of the Rhizaria are free‐living, forming important links in food chains, but others are parasites of algae, aquatic or terrestrial plants, or even marine invertebrates. Some marine Rhizaria (e.g. Radiolaria or Foraminifera) are mixotrophs, hosting a diversity of algal photosymbionts (e.g. green algae, dinoflagellates, etc.). Some testate Rhizaria, bearing mineralised skeletons, settle into aquatic sediments upon death and contribute to the palaeontological record of life and the environment over vast geological time periods.

Rhizaria in the oligotrophic ocean exhibit clear temporal and vertical variability

“…Foraminifera also had a bimodal distribution. Some lineages of Foraminifera are known to host photosymbionts (Biard, 2022a;Kimoto, 2015), however they are also efficient predators commonly seen throughout the mesopelagic (Caron and Be, 1984;Gaskell et al, 2019). Thus it is not 28 surprising to find their presence in both locations of the water column.…”

“…Some of the earliest records of their existence are from oceanographic expeditions in the 19th century (Haeckel, 1887). Rhizaria are unique members of the plankton and protist community because they can reach large sizes (up to several mm in diameter) and they construct intricate mineral skeletons out of either silica, strontium sulfate, or calcium carbonate (Biard, 2022a; Kimoto, 2015; Nakamura and Suzuki, 2015; Suzuki and Not, 2015). Despite their noticeable morphology and global distribution, Rhizaria were largely understudied throughout the 20th century.…”

Gelatinous matrix, an original strategy to cope with oligotrophy in Nassellaria (Radiolaria)

“…Among marine planktonic eukaryotic organisms, Radiolaria are diverse and highly abundant protists, widely distributed in the ocean, from tropical to polar waters [1, 2]. These protists exhibit a great range of lifestyles (solitary, colonial or with symbionts), trophic diversity (heterotrophic or mixotrophic) and tremendous morphological variability, spanning a wide range of sizes, from micrometers to several millimeters [3, 4]. Among the phylogenetic groups of Radiolaria, Acantharia [5] is the sole group of Radiolaria bearing a strontium sulfate skeleton, while the Polycystines group possesses an opaline silica skeleton [6].…”

Siliceous Rhizaria abundances and diversity in the Mediterranean Sea assessed by combined imaging and metabarcoding approaches

“…In the modern ocean, Rhizaria, a group of unicellular eukaryotic organisms that span a wide range of sizes, from tens to hundreds of micrometres, are known to play important roles in food webs and biogeochemical cycles (Biard, 2022). In the last decade, this group has been highlighted as a critical source of carbon export (Lampitt et al, 2009;Guidi et al, 2016;Gutierrez-Rodriguez et al, 2018), silica cycling (Biard et al, 2018;Llopis Monferrer et al, 2020), and a significant component of ocean biomass (Biard et al, 2016).…”